The present disclosure relates to a nozzle box assembly and, more particularly, to a nozzle box assembly provided to the inlet of the first stage of a steam turbine so as to inject the steam of high temperature and high pressure to the first stage.
A related art nozzle box assembly for a steam turbine, as shown in FIG. 1 and FIG. 2, typically includes three constituent elements, that is, a torus 14, a bridge ring 16 and a steam path ring 12. Each of the constituent elements is prepared as a 180° segment in the initial stage and then the constituent elements are welded in sequence so as to form two nozzle box halves 18. FIG. 1 and FIG. 2 show one of the nozzle box halve 18, wherein the other one, which is not shown, also has the same shape and structure.
Next, the two halves 18 are joined together along a horizontal center line so as to form a nozzle box assembly for a steam turbine. Each of the nozzle box halves 18 includes one or more steam inlets 10, which are integrally formed with the torus 14. The steam inlets 10 are connected to the torus 14 on a plane surface, which is perpendicular to the rotation shaft of the turbine.
During the operation of the steam turbine, steam from a steam supply source such as a boiler and the like is introduced through the steam inlets 10 and flows in the torus 14. The flow direction of the steam is typically changed to the axial direction such that the steam flows through the annular opening of the bridge ring 16 to the inside of the steam path ring 12. The steam path ring 12 is provided with a series of nozzles, including airfoil vanes 13 for directing the steam flow.
The related art nozzle box assembly as described above essentially includes a bridge ring 16 for connecting the torus 14 to the steam path ring 12. That is, it is necessary to interpose the bridge ring 16 between the torus 14 and the steam path ring 12 in order to connect the torus 14, which has an internal space in the shape of a circular cross-section, and the steam path ring 12, which is extended long in the rotation shaft direction of the turbine, with a smoothly curved surface. The smoothly curved surface connection formed by the bridge ring 16 smoothly induces the flow of the steam, which is introduced through the steam inlets 10, in a direction along the steam path ring 18, thereby serving to improve the flow efficiency.
As described above, the bridge ring has been applied in order to improve the flow characteristics of the steam, in which steam flow direction is rapidly changed. However, the bridge ring becomes a factor of the increase of a welding portion between the torus and the steam path ring, thereby resulting in the complicated manufacturing procedure while increasing manufacturing costs.